In continuous ink jet printing, electrically conductive ink is supplied under pressure to a manifold that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The filaments or individual droplets in the streams can be selectively charged by a charge plate and deflected from their normal trajectory. The deflected drops (streams) may be caught in a catcher and recirculated, and the undeflected drops (streams) allowed to proceed to a print receiving medium.
Charging is accomplished by a charge plate having a plurality of charging electrodes along one edge, and a corresponding plurality of leads along one surface of the charge plate. The edge of the charge plate having the charging electrodes is placed in close proximity to the ink jet filaments and charge is applied to the leads to induce charge on the filaments.
U.S. Pat. No. 4,560,991 issued Dec. 24, 1985 to W. Schutrum describes one method of fabricating a charge plate useful in an ink jet printer. In the method disclosed by Schutrum, the charge plate is fabricated by electro-depositing the charging electrodes and leads on a flat sheet of etchable material such as copper foil to form a so-called "coupon". The coupon is bent in a jig at approximately a 90.degree. angle, with the leads and electrodes on the inside of the bend. The leads and electrodes are then bonded to a charge plate substrate, and the etchable material removed. Electrical contact can be made to the leads in a number of ways including pressure contact in a socket, where the charge plate functions as a plug received by the socket, reflow soldering, and wire bonding. As connection densities exceed approximately 25 per inch, the plug and socket technique is no longer feasible. High density reflow solder connections are possible up to about 100 per inch. However, at densities higher than about 50 connections per inch solder crystals can grow after fabrication from one connection to a neighboring connection eventually causing shorts that may not become evident until weeks or months after manufacture. At densities between 50 and 100 connections per inch, there is some loss in yield using the solder technique. At densities greater than 100 connections per inch, there is a substantial loss in yield and reliability using the solder connection technique. Recently, ink jet print heads having 200 to 500 jets per inch requiring charge plates with 200 to 500 charging electrodes per inch are under development. While the electrode leads can be fanned out to achieve the low connection densities achievable by reflow solder connections, such a fan out can add significantly to the size of the lead pattern. The increased area occupied by the lead pattern can produce an undesirably large print head size and also increase the risk of defects, due to pinholes, etc., in the formation of the conductive leads. The reflow soldering technique used to make connections between these high density ink jet print heads and the connectors to the print heads requires a complicated series of manufacturing steps including selectively adding solder to the conductive elements of both the connector and the print head; removing excessive solder from the conductive elements, typically by scraping; applying a controlled amount of solder flux to the electrodes; carefully aligning the electrodes; and heating the solder to cause reflow under carefully controlled temperature and pressure conditions. Frequently, a repair to a faulty connection is required. This process creates several problems since the steps of applying and removing excess solder are very sensitive to operator error. Too much solder creates shorts between adjacent electrodes and too little solder and/or flux results in open circuits between the print head and the connector. These problems are in addition to the problem of growth of lead oxide crystals which over time interact to form shorts between the electrodes.
It is important to note that in the wider ink jet printing heads as illustrated by the 4 inch print head shown in U.S. Pat. No. 4,999,647, the number of connections to the print head are so high that a failure rate that would have produced a yield of approximately 85% in a 1 inch print head would produce a very small percentage yield in the 4 inch print head.